Knowledge Bank

We here theoretically outline and experimentally demonstrate that Polarization Spectroscopy can be combined with Cavity RingDown (CRD) spectroscopy, thereby retaining the specific advantages of both techniques. The $b1\sum g+(v\prime =2)\leftarrow X3\sum g-(v\prime \prime =0)$ transition of molecular oxygen around 628 nm is used to demonstrate the possibility to selectively measure either the polarization dependent absorption or the resonant magneto-optical rotation of gas-phase molecules in the appropriate set-up. Just as in CRD absorption spectroscopy, where the rate of absorption is measured, in the here presented Polarization Dependent CRD (PD-CRD) detection scheme the rate of polarization rotation is measured, which enables the polarization rotation to be quantitatively determined. Apart from studying electro-optic and magneto-optic phenomena on gas phase species, the PD-CRD detection scheme is demonstrated to be applicable to the study of magneto-optical rotation in transparent solid samples as well. Rotationally resolved spectra of the $b1\sum g+(v\prime =2)\leftarrow X3\sum g3(v\prime \prime =0)$ band of molecular oxygen are recorded by CRD spectroscopy in magnetic fields up to 20 Tesla. Measurements are performed in a short cavity, placed in the homogeneous field region inside a Bitter magnet. CRD absorption spectra are measured with linearly and circularly polarized light, leading to different $\Delta M$ selection rules in the molecular transition, thereby aiding in the assignment of the spectra. Frequencies and intensities of the rotational transitions of the oxygen A band in a magnetic field are calculated, and all observed spectral features are well reproduced. A discussion on the alignment of the oxygen molecules in the magnetic field is given.